This invention relates generally to the field of preparing taxol and taxol precursors. The process of this invention also produces new taxane intermediate compounds.
Taxol, a material occurring in nature, and extracted from Taxus brevifolia (i.e., the Pacific yew tree) and other biomass has been identified as having significant tubulin binding (Schiff, P. B., et al., "Promotion of Microtubule Assembly in vitro by Taxol," Nature, 277: 665-67 (Feb. 1979)) and, when delivered to the cell, cytotoxicological activity which has been demonstrated through Phase III clinical trials. Taxol has now been approved for treatment of refractory ovarian cancer by the U.S. Food and Drug Administration. Generally, taxol is only isolated on a large scale from the bark of Taxus brevifolia; unfortunately, the yield is relatively low even by the most efficient processes. The actual and potential demand for taxol far exceeds the supply currently available by extraction of taxol from natural sources. (Kingston, "The Chemistry of Taxol, Pharmac. Ther., Vol. 52, pp. 1-34, 5-6 (1991); "Kingston"). The process described herein could significantly increase the yield of taxol from these sources.
Taxol is a complex compound represented by the following formula: ##STR1## wherein reference numerals 2', 7 and 10 identify positions on the taxane ring significant to the nomenclature used herein.
Because of the physical and chemical complexity of the taxol molecule, the synthesis of taxol is extraordinarily difficult and has not been accomplished to date. "[I]t is . . . quite unlikely that a commercially feasible synthetic route to taxol will be developed before the end of this century." (Kingston at p. 24.) "Despite the progress made in [synthesizing taxol], the final total synthesis of taxol is, nevertheless, likely to be a multi-step, tedious, and costly process." (U.S. Pat. No. 5,015,744 at col. 1, lines 59 et seq.) The complexities of synthesizing taxol are evident from a cursory reading of Swindell, C. S. "Taxane diterpene synthesis strategies: A review." Org. Prep, Proced. Int. 23:465-543, 537 (1991) ("Swindell").
Even the partial synthesis of taxol from related compounds is quite difficult. "Taxol is the most functionally and stereochemically complex of the taxanes." (Swindell, at 467.) Among other things, the taxol molecule presents numerous reaction sites with similar chemical constituents in close proximity. This presents a problem, for example, with respect to any reaction attempting to affect any of the numerous oxygen substituents present at positions 1, 2, 4, 5, 7, 9 and 10 of the taxaneoring. (See, e.g., U.S. Pat. No. 4,876,399 to Holton et al., col. 3, lines 13-18.) This chemical complexity makes it difficult to direct reactions with significant specificity, except through the use of blocking agents and very controlled reaction parameters which favor a particular reaction at a particular site. Accordingly, yields of the desired product from reaction of taxol or taxol related compounds with a given reagent are frequently quite low.
In addition, the stereochemistry of the taxol molecule is considerably more complex than even the two dimensional formula depicted above. In fact, the taxol molecule has been characterized as "an inverted cup shape, in which the ester side chain lies across the opening of the cup." (Kingston at 3.) Kingston includes a more detailed two-dimensional depiction of taxol's stereochemistry.
As a result of these considerations, the chemistry of taxol and taxol related compounds is difficult and unpredictable.
Researchers have attempted to avoid some of these problems by focusing on the possibility of developing taxol related materials with tubulin binding and cytotoxicological activity. However, "with few exceptions, changes in the taxane skeleton appear to reduce the activity of taxol." (Kingston at 31.) Thus, the production of taxol is still generally preferred over other compounds with similar or analogous structures.
The current invention relates specifically to the partial synthesis of taxol and taxol precursors from taxanes containing a glycoside group at the C-7 position. (See the structural drawing above.) These glycoside taxanes are produced in nature and can be recovered with taxol during production from Taxus brevifolia. Glycoside substituted taxanes such as 10-deacetyl-7-xylosyl taxol (i.e., "10-DAXT") have been isolated along with taxol (see V. Senilh, et al., "New Derivatives of Taxol, . "J Nat Prod. 47:131 (1984); "Senilh et al."). The naturally-occurring 7-glycoside taxanes include the following specific materials: 7-xylosyl-10-deacetyl taxol A, 7-xylosyl-10-deacetyl taxol B, 7-xylosyl-10-deacetyl taxol C, 7-xylosyl-taxol A, 7-xylosyl taxol B, and 7-xylosyl taxol C. The structure of these compounds is represented by the following examples:
10-deacetyl-7-xylosyl taxol A: ##STR2##
10-deacetyl-7-xylosyl taxol B: ##STR3## and 10-deacetyl-7-xylosyl taxol C:
In addition, the process of this invention may be applied to 10-deacetyl-7-xylosyl baccatin III ("10-DAXB"): ##STR4##
Unless otherwise utilized herein, "taxol" shall collectively refer to the A, B and C variants. "Taxane" is utilized herein to refer to any compound having the characteristic cyclic structure repeated in the foregoing diagrams.
Typically, acid hydrolysis is used to remove, i.e., cleave, glycosides from chemical compounds to which they are attached. Common hydrolyzing agents include acetic or mineral acids in water/MeOH. Accordingly, hydrolysis of glycoside substituted taxanes has been suggested in the literature. (See Senilh et al. at 137.) However, replication of the experiment shown in Senilh et al. utilizing methanol and acetic acid as the hydrolyzing agent with 10-DAXT confirmed the decomposition of the taxane into numerous molecular fragments. The production of 10-DAT, i.e., the discrete cleavage of the glycoside from the taxane molecule, could not be observed. This indicates that the reaction shown in Senilh et al. is not selective and is consistent with the general observation that the taxol molecule is sensitive to strongly acidic hydrolysis conditions. (Kingston at 15, Section 3.6.)
It has not been previously known that it was possible to use taxanes containing a glycoside group at the C-7 position for the synthesis of taxol or taxol precursors. To date all partial synthetic routes for the production of taxol have derived from baccatin III, a significantly different starting material. (Kingston at 17.) Since baccatin III is not directly recovered from biomass, there are preliminary conversion steps to generate it as a "starting material."